Up till now, with demands for high-capacity and high-speed radio communication, techniques are actively studied for further improving use efficiency of limited frequency resources. In particular, techniques utilizing the space domain have attracted attentions. Adaptive array antenna (adaptive antenna) is an example of these techniques.
With this antenna, by adjusting the amplitude and phase using weighting coefficients (referred to as “weight”) by which received signals are multiplied, it is possible to strongly receive signals reached from a desired direction. Further, it is possible to suppress signals as interference components such as multipath interference and interference from the same channel. By this interference suppression effect, it is possible to improve communication capacity of communication systems.
Further, as other techniques utilizing the space domain, there are two techniques utilizing spatial orthogonality of channels. One of the techniques is a spatial multiplexing technique for transmitting different data sequences to the same terminal apparatus using physical channels of the same time, same frequency and same code. The following is a general example of using the spatial multiplexing techniques (e.g., see Non-Patent Document 1). That is, the transmitter and receiver each have a plurality of antennas. Further, it is possible to realize spatial multiplexing transmission in a propagation environment where the correlation of received signals is low between the antennas.
Here, upon transmission, from a plurality of antennas in the transmitter, different data sequences are transmitted on a per antenna element basis, using physical channels of the same time, same frequency and same code. Further, the antennas of the receiver demultiplex and receive the data sequences based on an estimation value (hereinafter “channel estimation value”) of channel characteristics. By this means, it is possible to make transmission processing fast by using a plurality of spatial multiplexing streams, without using M-ary modulation.
Further, when the transmitter and receiver each have the same number of antennas and perform spatial multiplexing transmission, it is possible to increase the communication capacity in proportion to the number of antennas, in an environment where the S/N ratio (signal to noise ratio) is sufficiently high and where there are many scatterers between the transmitter and the receiver. As the spatial multiplexing transmission scheme, a multicarrier modulation scheme using OFDM (Orthogonal Frequency Division Multiplexing) is likely to be used.
The reason for this is as follows. That is, if the multipath delay on a radio channel is within the guard interval time, a flat fading environment is identified in subcarrier units. Therefore, multipath equalization processing is not necessary, and demultiplexing processing of signals subjected to spatial multiplexing transmission is reduced.
On the other hand, upon reception, signals received by the receiver's antennas are frequency-transformed to baseband signals and further subjected to OFDM demodulation processing.
Here, the multicarrier modulation scheme is a transmission scheme using a plurality of subcarriers. Input data signals to the subcarriers are modulated into subcarrier signals by M-ary QAM modulation and such. OFDM, in which the frequencies of subcarriers are orthogonal, collectively transforms subcarrier signals of different frequencies using an FFT (Fast Fourier Transform) circuit.
By this means, after subcarrier signals are transformed into time domain signals, the transformed signals are frequency-transformed into carrier frequency bands and transmitted from antennas. OFDM modulation and OFDM demodulation are disclosed in Non-Patent Document 2.
Conventionally, in such a situation, channel estimation values are acquired by two-step channel estimation processing (e.g., see Patent Document 1). To be more specific, first, a received signal of a reference signal for channel estimation is divided per transmission antenna subset. Further, as the first step of channel estimation, the first step of channel estimation is performed based on the reference sequence. By this means, a tentative estimation value of the channel response in the first dimension (e.g., subcarrier direction) is calculated per transmission antenna subset using interpolation processing.
Next, as the second step of the channel estimation, a channel estimation is performed in a different dimension direction (e.g., time domain) using the tentative estimation value interpolated in the first dimension direction. Thus, a channel estimation value of a data part between a reference signal and a tentative estimation value is acquired by interpolation and channel estimation values of other data parts are acquired using extrapolation uniformly. By this means, it is possible to acquire a channel estimation value per antenna subset of transmission antennas.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-515481    Non-Patent Document 1: G. J. Foschini, “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas,” Bell Labs Tech. J., pp. 41-59, Autumn 1996    Non-Patent Document 2: Hiroshi Ochi, Kenji Ueda, “OFDM system technology and MATLAB simulation Guide,” Triceps, 2002